Fiber placement machines are well known in the art. Such machines are shown by U.S. Pat. Nos. 5,223,072 for an Intelligent Servo Controlled Fiber Placement Machine Tensioner, and 4,872,619 for a Servo Driven Redirect Roller Apparatus for Fiber Placement Machine. Such machines are used to lay fiber tows onto a form or tool in order to construct a fiber composite part. Such parts are used in the aircraft and aerospace industries because of their high strength and low weight. Fiber placement machines use a creel to supply fiber tows to a fiber placement head which applies the fiber to the tool. A fiber path is provided between the creel and the head to allow fiber bundles to be fed from the creel to the head without overlapping one another or becoming abraded in the guides or rollers of the machine. The fiber placement head is mounted on the end of an arm and a wrist joint is provided between the arm and head to allow the head to swivel to the desired position in order to lay the fibers on the tool.
Three sets of rollers are positioned in the wrist to control the path of the fiber tows between the end of the arm and the fiber placement head and to maintain the fibers in the proper position relative to one another. The J-axis rollers are located along the middle section of the wrist and direct fibers coming from the lower part of the wrist to the end of the wrist that is opposite the fiber placement head. The A-axis rollers are located at the end of the wrist that is opposite the fiber placement head and are used to reverse the direction of the fibers coming from the J-axis rollers so that the fibers are directed to the head. The K-axis rollers are located at the end of the wrist adjacent the head and direct the fibers from the A-axis rollers to the fiber handling mechanism in the head.
The wrist joint is capable of rotary motion about the X, Y, and Z axes of the machine. Depending on the shape of the part and the stresses to which the laid-up part will be subjected, the tape head is oriented to a selected angular position for each of its passes over the tool in order to place the fiber on the tool in the desired orientation.
Recently, applications have arisen for fiber placement machines that require the head to rotate 155 degrees to either side of a Home-position, or 310 degrees in total. Such extreme rotary motion of the head places demands on the fiber delivery system and can cause adjacent tows to rub against one another and chafe, or curl and roll onto themselves as they pass over the rollers.
The rotary motion of the head also changes the path length of the fibers from the creel to the head, and tensioners are used on each of the spools in the creel to compensate for path length change. The tensioners must pay out or rewind the material on the spools in the creel in order to maintain the proper tension on the tows as the fiber placement head moves through different positions and orientations. The less the path length changes, the less manipulation is required by the tensioners in the way of acceleration, deceleration and reversal of the spools.
Minimizing path length change also improves other problems associated with having the tensioners rewind material onto the spools. The first time the fiber is payed out from the spools, a backing film that is coated with a release material is stripped off. If the fiber then has to be rewound onto the spools, it is rewound without any release material in between adjacent layers, causing the fiber to stick to itself.
Additionally, the original “spiral” winding of the fiber on the spool cannot be duplicated when rewinding. As a result, rewound material will tend to be stacked upon itself, increasing the possibility of roped tows or clumping.
Each of the above problems associated with changing the path length of the fibers is exacerbated if the width of the fiber band is increased, and in order to meet increased production rates, the width of the fiber band in modern machines is twice that of older machines.
Accordingly, it would be desirable to provide a fiber placement machine with a fiber tow path including J, A, and K-axis rollers in the wrist that would allow increased manipulation of the fiber placement head without damaging the tows and while keeping the changes in fiber path length to a minimum.